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Name:____________________________ Section:____________________ Earth 110 – Exploration of the Solar System Assignment 3: Planetary Geology Due in class Thursday, Feb. 5, 2015 Planetary geology is the study of Earth extended to other worlds. Most of what we know about planet interiors comes from the study of Earth’s interior. The composition and structure of planetary interiors is extremely important when trying to piece together how the planet evolved through time. A number of datasets can be collected that, when combined, give scientists a pretty detailed picture of what’s going on. Seismological data provides the only detailed cross-section of planetary interiors, and we currently only have these datasets for the Earth and the Moon (this study also depends on quakes actually occurring). The average density of planets allows scientists to infer simple interior structure based on what is known about the surface. Spacecraft data, particularly from orbiters, contributes additional constraints on mass and density distribution. Samples of the shallow interior come from volcanic rocks, while meteorites supply samples of protoplanets that scientists believe our planets evolved from. The presence of a magnetic field indicates an electrically conducting fluid inside the planet, or magnetized rocks in the absence of a magnetic field suggests the planet once had a fluid layer. Ultimately, what takes place inside a planet affects what happens on the surface. A main component of planetary geology is comparative planetology. Scientists can take what they know about geological processes on one planet and compare them to other planets in order to learn how the surfaces are modified. This is particularly important when trying to determine how old or active a surface is, or what conditions may have been like millions or billions of years ago. Surface features are windows into a planet’s past. However, atmospheres erase this past, as is the case for Venus and Earth. This assignment includes material from Chapter 9 of the textbook and we will be focusing on the terrestrial worlds (inner solar system). We are getting into a more qualitative part of geology (for this class, at least!) since we are focusing on processes that involve relative time, such as heat generation/loss, and large-scale structure of planets. Surface comparisons rely heavily on visual analysis, so keep your eyes open!

Terrestrial planets of our solar system

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Planetary Interiors Seismology:

Seismology is the study of earthquakes and the propagation of waves (or vibrations) generated by earthquakes. There are two main types of waves that travel through the Earth’s interior: P – or pressure (also primary) – and S – or shear (also secondary) – waves. P wave velocities are always faster than S wave velocities, so P waves will arrive at a location before S waves.

P waves result from compression and extension in the direction the wave is traveling in (hence the pressure term). An important property of P waves is that they can travel through almost any kind of material (solid, liquid, or gas) because molecules can always push on their neighbors.

S waves result from up-and-down or side-to-side motion that is perpendicular to the direction the wave is traveling in (hence the shear term). S waves only travel through solids because liquids or gases cannot be sheared (the bonds between neighboring molecules are too weak to transfer the up/down or side/side motion).

In the diagram above, label which wave is P and which is S.

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The speed and direction of seismic waves are dependent on composition, density, pressure, temperature, and phase (solid or liquid) of the material they travel through. Because of this, seismic waves can provide a fairly detailed look into the interior structure of a planet.

Check out the graph below and label these layers by looking at the velocity profiles: Lithosphere (rigid rock, the crust and part of upper mantle), Mantle, Outer Core, Inner Core. Explain why you placed the layers where you did. Are the layer boundaries sharp or gradational? Can you say anything about the phase (solid or liquid) of the material by looking at the wave velocities?

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Average Density: Find the average density (in kg/m3) for each of the terrestrial planets (including the Moon):

Planet   Mass  (kg)   Radius  (km)   Density  (kg/m3)  

Mercury   3x1023   2439    

Venus   5x1024   6051    

Earth   6x1024   6371    

Moon   7x1022   1737    

Mars   6x1023   3389    

Crustal rocks are mainly composed of granitic and basaltic rocks with densities up to 2700 kg/m3 and 3000 kg/m3, respectively. Compare these average densities with those of the planets. What can you say about the interior composition and structure of each planet? Based on size and density, which of the planets are anomalous?

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Generating and Losing Heat: Heat is the ultimate driver of geologic activity, both on the surface and inside a planet. But where does this heat come from? Describe, in your own words, how the below processes generate heat: Accretion: Differentiation: Radioactive Decay: Have all of these processes generated heat over Earth’s entire lifetime? Which are most important today? Which were most important when Earth first formed? Which process is responsible for the layers inside terrestrial planets?

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Just like there are three ways that planets gain heat, there are three ways they lose heat. Smaller planets lose this heat faster than larger planets because larger objects have a smaller surface area-to-volume ratio (check out pg. 239 in the textbook). Describe, in your own words, how the below processes transport heat. Conduction: Convection: Radiation: In the diagram below, show where each of these processes of heat transfer is occurring for Earth (Hint: it takes about 100 million years for a rock to be transported from the base of the mantle to the top, but a rock at the base of the crust more or less stays at the base).

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Planetary Surfaces There are four main processes that shape planetary surfaces: impact cratering, volcanism, tectonism, and erosion. Volcanism and tectonism are governed by a planet’s internal heat, while erosion requires an atmosphere. Retaining an atmosphere depends on the planet’s gravity and the amount of volcanic outgassing. The formation of craters is random and not controlled by planetary properties.

Impact Cratering: Smaller impactors form simple craters that are bowl-shaped. Larger craters that have central peaks or rings around the rim are complex craters. Scientists use the number of impact craters on a surface to determine its relative age because the longer a surface has been exposed, the more impact craters it should have.

Check out the below image of the Moon: what can you say about the relative ages of the lighter and darker areas? Are most of the craters simple or complex? Are most of them fresh (sharp rims) or old (rounded rims, shallow floors)?

How does the relative age of the surface in the Mars image (below) compare with that of the Moon image? What do you notice about the shape/depth of the craters?

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Volcanism: The result of lava eruption depends on how easily it flows across the surface, forming three types of volcanic features. Lava that is runny will flow farther and form volcanic plains. Thicker lavas will solidify before they can flow very far, forming shield volcanoes (like Hawaii) that have very shallow slopes. The thickest lavas solidify very close to their eruption site and build very tall, very steep stratovolcanoes (like Mount Fuji or Mount Hood). Label which type of volcanic feature is shown in each image below.

Volcanism on Mercury

Volcanism on Earth

Volcanism on Venus

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Tectonism: Tectonics build surface features through compression, extension, or other forces acting on the lithosphere. On Earth, the lithosphere is broken up into more than a dozen plates, and the movement of these plates (plate tectonics) is responsible for building continents and recycling ocean floor. The other terrestrial planets do not have plate tectonics – scientists call them one plate planets. Regardless of single or multi-plate surfaces, all planets show signs of tectonic activity. Compression results in mountains, scarps, or ridges while extension results in cracks or valleys. Lateral motion – like the San Andreas Fault in California – offsets preexisting features. What type of tectonic motion is represented in the below image of Mars? How do you know (Hint: what would happen to features if they were subjected to the same forces?)?

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Below is a picture of Mercury. What type of tectonic motion is shown (arrows)? How do you know?

Below is an image of two craters on the Moon. What tectonic motion do you observe? When was the relative timing of this motion (Hint: compare to when the craters would have formed)?

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Erosion: Erosion is the transport of material through the action of ice, liquid, or gas. Erosion breaks features down, like mountains and valleys, but also builds features like sand dunes and river deltas. In fact, sedimentary rocks, like sandstone and shale, are formed from other pieces of rock that have been eroded and re-deposited. Below are some images of crescent dunes on Earth and Mars. Which picture is Earth and which is Mars? How can you tell?

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Below are some images of water carved channels on Earth and Mars. Which picture is Earth and which is Mars? How can you tell? Today, liquid water is not stable on the surface of Mars because pressures and temperatures are too low. What can you deduce about Mars’ past? What could have caused this change?